The fabrication of printed-circuit board assemblies is now highly automated. The printed-circuit boards are pre-drilled for the component leads, and automatic insertion of the component leads ("stuffing" the board) is commonplace. In order to keep the components from falling out of the printed-circuit board after the component leads are inserted, if soldering is not performed immediately after the lead insertion, the leads may be clinched by bending over that portion of the leads protruding from the opposite side of the printed-circuit board. The lead clinching operation is often combined with the component lead insertion step. After the lead clinching, or in conjunction therewith, the excess lead length is cut off. After the clinching/cutting step, the printed-circuit board, now loaded with components, may be ready for soldering or fusion of the leads to terminals of the printed-circuit board. The terminals are often co-located with the lead-accepting holes.
The soldering of leads was performed in the past by applying soldering paste or other oxidation control material to the underside of the board, and passing the loaded printed-circuit board through a solder wave. The solder wave heated the leads and the terminals, and if the gap therebetween was small, left an adherent mass of molten solder, which quickly cooled and solidified when outside the solder wave, to form the desired fused connection. The solder wave provides a great time and cost advantage over individual soldering of each terminal to each lead.
Soldering of leads to their terminals by the use of solder waves is subject to a number of disadvantages. One disadvantage is the environmental effect of large amounts of molten lead-containing solder. Another disadvantage is the difficulty of maintaining an even solder wave, which does not extend too high or too low at any position along the width of the printed-circuit board being soldered; this problem is exacerbated by the presence of dross, which is nonsoluble material which tends to float on the solder, and arises for the most part due to oxidation of flux materials. A major disadvantage of the solder-wave technique is that the high density of the solder wave tends to cause the components to float on the wave, and results in components which are soldered in positions which are not anticipated, high above the surface of the component side of the printed-circuit board. The clinching operation is intended as much to prevent the components from floating completely out of the board, which might result in closing the lead-accepting hole or aperture with solder, without the lead being in the hole, which would require rework to re-melt the solder in that particular hole, and insertion of the component at the same time. Such rework is expensive, but what is worse, the reworked solder joint is less reliable than one which is virgin.
As a result of the disadvantages of the solder-wave soldering technique, the solder reflow soldering method has become common. In the solder reflow method, the lead side of the printed-circuit board is coated with solder paste, which includes flux-type material. The printed-circuit board is then "stuffed" with components, and the leads are clinched, if necessary, and cut to length. The stuffed printed-circuit board, or at least the portion to be soldered, is then placed in a high-temperature environment long enough to cause the solder to "reflow" and make the fused joint. The board is then removed from the heated environment, or the heated environment is removed from the board, and the fused joints solidify to complete the soldering operation. The reflow soldering method has major advantages in control of the amount and location of the solder, in environmental problems, and in the problem of improperly oriented or located components due to floating.
Improved methods for placement of components in printed circuit boards in conjunction with soldering or fusion joining are desired.